1. Technical Field
The present invention relates to a normally-off Hetero Field Effect Transistor (HFET) and a method of manufacturing the HFET.
2. Description of the Related Art
A related-art HFET includes: a semiconductor substrate made of SiC (or Si, GaN, sapphire or the like); a buffer layer made of AlN formed on the semiconductor substrate; an electron transit layer made of non-doped GaN formed on the buffer layer; an electron supply layer formed on the electron transit layer and made of a non-doped AlGaN layer or laminated layers including a non-doped AlGaN; an insulating film made from SiOx (x is an integer from 1 to 2), which is formed on the electron supply layer and a part of which is opened; and a gate electrode, a source electrode, and a drain electrode which are formed on the electron supply layer. Here, the term “non-doped” means that an impurity is not intentionally introduced into a semiconductor layer.
A band gap of AlGaN is larger than that of GaN, and a lattice constant of AlGaN is smaller than that of GaN. As a result, when the electron supply layer made of AlGaN is formed on the electron transit layer made of GaN, a tensile stress is acted on the electron supply layer and piezoelectric (voltage) depolarization occurs. Since spontaneous depolarization also occurs in the electron supply layer, an electric field caused by the piezoelectric depolarization and the spontaneous depolarization is acted on a heterojunction interface between the electron transit layer and the electron supply layer, and thus a carrier layer called as a two dimensional electron gas (2DEG) layer occurs. The 2DEG layer is used as a channel, and thus the HFET is used as a switching element which can control electron flow from the drain electrode to the source electrode through the channel therebetween.
On the other hand, in the HFET, since an energy level in the heterojunction interface between the electron transit layer and the electron supply layer is equal to or less than the Fermi level, a normally-on (depletion mode) characteristic of a negative threshold value appears. However, for example, in a semiconductor device which is applied to a power supply apparatus as a power semiconductor element, it is necessary to be a normally-off (enhancement mode) type of a positive threshold value for ensuring the safety in a malfunction. JP-A-2006-339561 describes a field effect transistor of normally-off type.
FIG. 6 is a cross-sectional view illustrating the HFET which uses a GaN-based material and has the normally-off characteristic. The HFET includes: a semiconductor substrate 101 which is made of SiC; a buffer layer 102 formed on the semiconductor substrate 101 and made of AlN; an electron transit layer 103 formed on the buffer layer 102 and made of non-doped GaN; an electron supply layer 104 formed on the electron transit layer 103 and made of non-doped AlGaN; a p-type semiconductor layer 105 formed on a part of the electron supply layer 104 and made of a p-type GaN; a high concentration p-type semiconductor layer 106 formed on the p-type semiconductor layer 105 and made of a high concentration p-type GaN; an insulating film 107 made of SiOx and formed on the electron supply layer 104, side surfaces of the p-type semiconductor layer 105, and upper surface and side surfaces of the high concentration p-type semiconductor layer 106, and a part of the insulating film 107 is opened; a gate electrode 108 made of Pd, formed on the high concentration p-type semiconductor layer 106 and is in ohmic contact with the high concentration p-type semiconductor layer 106; and a source electrode 109 and a drain electrode 110 made of Ti and Al, being separately formed on the electron supply layer 104 so as to interpose the p-type semiconductor layer 105.
Since the p-type semiconductor layer 105 made of the p-type GaN is formed on the electron supply layer 104 immediately below the gate electrode 108, the energy levels of the electron transit layer 103 and the electron supply layer 104 are increased, and thus the HFET having the normally-off characteristic is obtained.
In order to more easily obtain a normally-off property in the HFET having the p-type gate structure, it is necessary to form the electron supply layer to be thinner, or to lower an Al combination ratio (mole fraction) in AlGaN which constitutes the electron supply layer. The both of the methods described above can easily provide a normally-off property, but reduce carrier concentration of the 2DEG layer. Therefore, the on-resistance is increased.